Respiratory protective apparatuses utilizing powered filtering devices or turbo filtering devices are known. In these devices air is delivered to a facepiece by a powered blower which is normally worn by the wearer using a body harness. The device may be connected to the facepiece by a breathing hose.
Powered filtering devices in some measure responsive to a wearer's demand are also known. For example, GB 2 032 284 discloses a respiratory breathing apparatus including a detector means for detecting exhalation by the wearer connected to a control means for at least reducing the flow of air through the filter means and flowing to the wearer during at least part of each exhale part of the breathing cycle of the wearer.
Such known devices, however, suffer from a number of problems and disadvantages. For example, in the device described in GB 2 032 284, the detector means is positioned at or near an inlet to a hood or face mask, remote from the control means. It must be connected to the control means by an electrical cable which must pass through the flexible breathing hose. The flexibility of the breathing hose, however, can cause the electrical cable to become weakened and liable to failure during use.
Another problem with known powered filtering devices is that they tend to be wasteful because they deliver air to a wearer when the wearer has no need of such air. This unnecessarily consumes filtration capacity and causes discomfort to the wearer.
Partially demand response devices, such as disclosed in GB 2 032 284, go some way to mitigating this problem. However, these devices still waste valuable electrical energy by overworking the device.
A further disadvantage of many known powered filtering devices is that they provide no measurement of air flow. As a result, a wearer is not provided with any warning that the air flow rate through the device has fallen below a minimum safe set level. Such a situation could easily occur due to filter clogging and the wearer needs to be advised of it in a timely manner.
It would be desirable, therefore, to have a powered filtering device which did not have these problems and disadvantages.